The present invention relates to a coated hard alloy tools used as a cutting tool that is required to show high wear resistance or other wear-resistant tool, and more particularly to a tool formed from a hard alloy and coated with a multilayered ceramic film which has good resistance to peeling and chipping.
Conventional steel cutting tools were made of cemented carbide (WC-Co alloy with carbonitrides of Ti, Ta or Nb added). However, in order to cope with increasing cutting speeds, most of such tools are nowadays formed from a matrix of a cemented carbide, a cermet or a ceramic material such as alumina or silicon nitride having its surface coated by CVD or PVD process with a 3-10 micron thick film of a carbide, nitride, carbonitride, carbooxide, boron nitride or oxide of a metal belonging to the IVa, Va or VIa group in the periodic table or Al, or their solid solution.
First-generation such coated tools had mainly a coating of a titanium compound of high hardness so that they showed high wear resistance at their rake face. But as the cutting speed of the tools increases, in order to reduce crator-like wear formed in the rake face, such a coating was gradually replaced by a triple-layer coating consisting of a lowermost titanium compound layer, an intermediate oxidation-resistant Al.sub.2 O.sub.3 layer, and a top gold TiN layer which is used to distinguish used corners from not-used ones. Most of the present-day tools have a coating of this type. In particular, it is now considered essential that such a coating include an oxide layer such as Al.sub.2 O.sub.3.
By increasing the thickness of the coating or by providing an Al.sub.2 O.sub.3 layer in the coating, the wear resistance of the tools improved. But their resistance to chipping decreased. This is presumably because the coating itself is a brittle material and thus it is likely to suffer cracks that extend the entire thickness of the coating. Another reason is that by providing a layer of Al.sub.2 O.sub.3 in the coating, the surface roughness of the coating increases. Thus, in order to improve the resistance to chipping, Unexamined Japanese Patent Publication 55-150941 and Examined Japanese Patent Publication 5-9201 proposed to reduce the thickness of the coating only at the ridge of the cutting edge. For the same purpose, Unexamined Japanese Patent Publications 62-228305 and 5-57507 proposed to limit the surface roughness of the Al.sub.2 O.sub.3 layer below a predetermined value.
The solutions proposed in these publications were effective to some degree in improving the resistance to chipping, but could not improve the wear resistance of the tool and particularly the peel resistance of the film coating. The tool can be damaged easily once their coating layer peels. Thus, the life of such a tool was short. A tool made of a hard alloy and having a coating which is less likely to peel has been long awaited.
It is an object of the present invention to provide a coated hard alloy tool that exhibits high resistance to chipping, wear and peeling in a balanced manner, and thus reveals high performance in cutting metals, and that shows long life.